Okay, so for the first time, I'm going to be the devil's advocate. Thank God the devil in this situation is microsoft 🤣 🤣. I have a GPD Win 4. Even the 6800u version. The following were improvements, be it on my device, when Xbox full screen experience was active:
Biggest improvement i saw was in heat!! Specifically the cpu, but really, the whole device. I can max my TDP (30w), turn cpu boost ON (i boost only 95% using quickCPU) and run borderlands 4 (NOTORIOUSLY demanding game) and temps never go above 80c. Hit 90+ even just idling or playing in desktop.
Fps actually went up on borderlands 4. In 50s on. 40s off
And games launch quicker, even if I have to hold xbox button to select window. As well as, cloud gaming just works more often. Less hit and miss. I've only ever been on windows, mainly because of simplicity with my set up, so, I can't and won't bag on linux based systems, but in my experience, the full-screen experience basically delivered what it said it would.